Internal Syphon PSI

Pat P.

On the side of my boiler is a pressure gauge labeled "Internal Syphon". It reads out pressures between 0 PSI and 30 PSI. I have my pressuretrol set at 0.5 cut-in and 1.5 cut-out. I have a pressure release valve that's rated for 15 PSI.
What exactly does the Internal Syphon gauge report on? If its the pressure in the boiler, shouldn't it trip the relief valve if the needle exceeds 15 PSI? How is it that needle can read more than the cut-in or cut-out pressure in a closed system. I am concerned that my pressure readings on the Syphon gauge are excessive.